WASHINGTON --- Turkey has begun building a platform site for a Russian S-400 missile defense system, despite pressure from Washington to abandon plans to purchase the military equipment, CNBC reported on Thursday, citing a source with firsthand knowledge of an unpublished intelligence assessment.

The source said the assessment was conducted about a month ago and includes satellite imagery of a concrete launch facility and other constructions, including bunkers, the US media reported.

The source also indicated that the work at the site "fits the pattern" for Russia's S-400 system, the report said. The report gave no indication about where the site is located in Turkey.

US officials have warned that if Ankara goes ahead with plans to purchase the Russian S-400 anti-missile systems, the United States could withhold the transfer of F-35 jets. The US Congress also recently passed legislation that would block the transfer of F-35s if Turkey buys the Russian S-400s.

In December 2017, Russia and Turkey signed a loan agreement to supply S-400s to Ankara. According to a statement by the Turkish defense industry secretariat, two S-400 batteries will be operated and serviced by the Turkish military.